Dichloridobis(2-{1-[2-(1H-indol-3-yl)ethyliminio]ethyl}phenolate-κO)zinc(II)–2-{1-[2-(1H-indol-3-yl)ethyliminio]ethyl}phenolate (1/2)

In the mononuclear complex molecule of the title compond, [ZnCl2(C18H18N2O)2]·2C18H18N2O, the Zn atom, which lies on a twofold rotation axis, is coordinated by phenolate O atoms in a tetrahedral coordination geometry. The coordinated Schiff base uses its indole NH donor site to form a hydrogen bond to the negatively charged phenolate O atom of the uncoordinated zwitterionic Schiff base. There is an intramolecular N—H⋯O hydrogen bond in the coordinated and uncoordinated Schiff bases. The indole NH site of the uncoordinated Schiff base does not engage in a hydrogen-bond interaction. The CH2—CH2 group in the uncoordinated Schiff base is disordered equally over two positions.

In the mononuclear complex molecule of the title compond, [ZnCl 2 (C 18 H 18 N 2 O) 2 ]Á2C 18 H 18 N 2 O, the Zn atom, which lies on a twofold rotation axis, is coordinated by phenolate O atoms in a tetrahedral coordination geometry. The coordinated Schiff base uses its indole NH donor site to form a hydrogen bond to the negatively charged phenolate O atom of the uncoordinated zwitterionic Schiff base. There is an intramolecular N-HÁ Á ÁO hydrogen bond in the coordinated and uncoordinated Schiff bases. The indole NH site of the uncoordinated Schiff base does not engage in a hydrogenbond interaction. The CH 2 -CH 2 group in the uncoordinated Schiff base is disordered equally over two positions.

Related literature
For a related neutral Schiff base, see: Rodriguez et al. (1987). For a related but zwitterionic Schiff base, see: Ali et al. (2007). For zinc derivatives of such deprotonated Schiff bases, see: Ali et al. (2008); Chen et al. (2007).

Experimental
Crystal data [ZnCl 2 (C 18 (Rodriguez et al., 1987) furnishes the expected zinc complex with the deprotonated ligand in which the deprotonated ligand N,O-chelates to the metal center (Chen et al., 2007). Similarly, the reaction of zinc acetate with the 4-methyl substituted Schiff base (the methyl substituent is para to the negatively-charged phenoxy group) affords the corresponding tetrahdral zinc compound; the structure of the Schiff base itself is not known.
On the other hand, the 4-methyl substituted Schiff base of 2-[2-(1H-indol-3-yl)ethylimino-1-ethy]phenol exists in the zwitterionic form; the presence of methyl group on the imino -C=N-double-bond probably induces charge separation (Ali et al., 2007). 2-[2-(1H-Indol-3-yl)ethylimino-1-ethy]phenol in the title cocrystal is a zwitterionic species; it coordinates to zinc chloride, so that the coordination geometry at the metal center is an Cl 2 O 2 Zn tetraheron. The compound crystallizes as a cocrystal (Scheme I, Fig. 1). Both the coordinated and free zwitterionis have an intramolecular N···O hydrogen bond. The coordinated Schiff base uses its N-H indolyl donor site to form a hydrogen bond to the negatively-charged phenolato-O atom of the free Schiff base. The N-H indolyl site of the free Schiff base does not engage in a hydrogen-bonding interaction.

Refinement
The ethylene linkage in the free Schiff base is disordered over two position; these were arbitrarily assigned 0.5 site occupancies; the temperature factors of the primed atoms were set to those of the unprimed atoms. The N3-C27 and N3-C27' were restrained to within 0.01 Å of each other; the four C-C bonds were restrained to 1.50±0.01 Å.
Carbon-bound H-atoms were placed in calculated positions (C-H 0.93 to 0.98 Å) and were included in the refinement in the riding model approximation, with U(H) set to 1.2 to 1.5U(C). The amino H-atoms were located in a difference Fourier map, and were refined with an N-H distance restraint of 0.86±0.01 Å; their temperature factors were freely refined. 2C 18 H 18 N 2 O at the 70% probability level; hydrogen atoms are drawn as spheres of arbitrary radiius. The symmetry-related zwitterionic Schiff base is not shown. Dashed lines denote hydrogen bonds. The mononuclear molecule lies on a twofold rotation axis.